Application example of two standard batteries compared

A standard cell is an electrochemical cell used as a voltage reference standard in many electrical standards laboratories. If properly cared for, standard cells are very stable. The current value of a single cell voltage is calculated from a series of measured voltage differences compared to a known reference standard.

Since the voltage difference between individual cells may be only a few microvolts, making precision measurements requires the use of a nanovoltmeter and low voltage measurement techniques. The following application examples illustrate comparing two standard cells and comparing a standard cell to a precision voltage reference standard.

Comparing two standard batteries

Comparison of standard cells requires measuring the potential difference between a reference standard cell and an unknown standard cell. The potential difference of each cell is determined by connecting them in a series reverse configuration. As shown in Figure 4-37, the negative terminals of standard cells V1 and V2 are connected together. Copper conductors are used to connect the two cells to the voltmeter to minimize errors caused by thermal electromotive force (VEMF).

After connecting the circuit, pay special attention to avoid errors caused by thermal EMF. To minimize the effect of thermal EMF, reverse the battery and make a second measurement, as shown in Figure 4-38. The absolute values ​​of the two readings are averaged to calculate a very small voltage difference.

During the entire comparison process, we hope to obtain a stability indicator of the voltage difference of the measured battery by calculating its standard deviation from several redundant readings.

Once the stability index is obtained, the voltage of each cell can be calculated from the group average. Several readings are usually averaged for each comparison. This intercomparison process is repeated regularly according to the time intervals established by the standard laboratory. The results of the measurements can be plotted and compared over time. This process is very useful for maintaining voltage values ​​of less than 6 cells. If more cells are to be maintained, an automatic scanner controlled by a computer can be used for more efficient management.

Comparison of Precision Voltage Source and Standard Battery

The voltage value of a precision DC voltage source can be determined using a standard battery, as shown in Figure 4-39. Use a precision voltage divider to roughly divide the voltage of the voltage source to the voltage of the standard battery. Use a nanovoltmeter as a zero detector to determine the voltage difference between the voltage divider output and the standard battery. As long as the voltage divider ratio and the voltage value of the standard battery are known, the voltage value of the precision DC source can be determined. Be careful not to draw any current from the standard battery, as this will cause the voltage of the standard battery to drift.

The output impedance of the voltage divider may generally be higher than the impedance of a standard battery, so the HI terminal of the nanovoltmeter must be connected to the output of the voltage divider, as shown in Figure 4-39, to avoid the common-mode current from causing additional voltage drop on the voltage divider resistor.